In general, a receiver in a wireless communication system does not have a priori knowledge of the physical channel over which the transmitted signal propagates or the time at which a transmitter transmits the signal. Timing synchronization, also known as clock recovery, is the process by which the receiver processes a received signal to determine the precise transition points within the received waveform. In other words, the receiver attempts to “synchronize” or align its clock with the clock of the arriving waveform. This process requires the receiver to estimate or otherwise determine the appropriate “timing offset” of the received signal, i.e., the amount of skew between the transmitter's clock and that of the arriving waveform.
Incorrect determination of the timing offset can have detrimental effects on other receiver operations, such as channel estimation, symbol detection, and the like. For example, an incorrect timing offset may cause the received waveform to be sampled at times during which the waveform is in transition between two symbols resulting in an increased number of symbol detection errors.
In general, timing synchronization in wireless communication systems typically comprises “peak-picking” the output of a sliding correlator between the received signal and a transmit-waveform template. The receiver uses the output of the sliding correlator to estimate the timing offset.
Ultra-wideband (UWB) systems transmit information via baseband transmissions with high penetration capability and rich multipath diversity that can be collected with low-complexity RAKE reception. However, the information-bearing waveforms are impulse-like and have low power, which increases the difficulty in achieving accurate and efficient timing synchronization. Additionally, peak-picking the output of a sliding correlator between the received waveform and the transmit-waveform template is suboptimum in the presence of dense multipath, and results in high receiver complexity with slow acquisition times.
Existing sliding correlator timing synchronization techniques operate with assumptions that are unrealistic in UWB systems. More specifically, many techniques require unrealistic assumptions such as an absence of noise or other interference, absence of multipath, and a priori knowledge of the communication channel. For example, one technique, referred to as coarse bin reversal searching, operates under the assumption that there is no noise. Coarse bin reversal searching estimates the arrival time of the received waveform by dividing each symbol duration into thousands of bins and searching for the bin containing the greatest energy with a sliding correlator. Another technique uses a coded beacon sequence designed in conjunction with a bank of correlators to operate in the absence of multipath. The coded beacon sequence is cross-correlated with the received waveform and estimates the location of the beacon sequence within the received waveform via peak-picking. The location of the beacon sequence is used to estimate the timing offset. Non-data-aided, i.e. blind, timing synchronization techniques operate in the presence of dense multipath but require that there is no time-hopping (TH) within each symbol. Such blind timing synchronization techniques use cross-correlation and rely on dense multipath and the cyclostationarity that arises from the time-hopping restriction. So called “ranging systems” do not use a sliding correlator, but rather use a priori knowledge of the strongest path of the communication channel to estimate the distance between transmitter and receiver.
Consequently, such conventional timing synchronization techniques do not operate functionally in realistic UWB environments. Moreover, many of these conventional timing synchronization techniques for UWB systems, especially those that utilize a sliding correlator with a transmit-waveform template, result in slow acquisition and high receiver complexity.